rockchip: rk3288: provide ${fdtfile}

All rk3288 default configs define CONFIG_DEFAULT_DEVICE_TREE.
So we can use it to define ${fdtfile} in rk3288_common.h.

This variable is needed by the distro boot command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>